So I was in a DF group with a friend where I encountered a player who did nothing but complain and berate the two of us most of the time. I have no problem owning my mistakes and always see them as an opportunity to improve my game. However, the only advice the player in question offered when confronted about his/her negativity was "stop sucking."

How does this actually help anyone get better at the game? More importantly, how does that help other people enjoy the game more? I feel like if you're going to speak about a problem with the way another player is playing the game, you ought to have something to say that will help that player improve. Otherwise, feel free to abandon the duty and find your own people to play with. In fact, if you're going to be that guy then you have nothing to complain about. The situation will stay the same throughout the entire duty, and you'll be just as negative at the end as you were in the beginning if not more so.